1. Field of the Invention
This invention relates to an apparatus and method for determining the quality of a fluid, such as engine or transmission oil. More particularly, this invention relates to a method and apparatus for determining the quality of oil using turbidity sensing technology.
2. Description of Related Art
The rising cost of highly complex engine and transmission repair, for example, in the earth moving equipment business, has generated a market interest for affordable transmission oil sensing. There has always been a significant interest in measuring oil quality to gain better control of viscosity breakdown and contamination from water or antifreeze or contaminants such as metal, dust, carbon, etc. The use of low cost oil quality sensing from a preventive maintenance perspective is highly desirable for reducing expensive equipment upkeep costs and obtaining better control of preventive maintenance scheduling.
Turbidity sensors are used in many different types of applications. Some turbidity sensors are used in association with machines for washing articles, such as dishwashers and washing machines. Most turbidity sensors measure the effect on a light beam by particulate matter suspended within a fluid. Some turbidity sensors use only a transmitted light signal while others use both scattered and transmitted light signals.
U.S. Pat. No. 4,198,161 to Larson teaches a low turbidity nephelometer which measures the turbidity of a water sample by directing a beam of light into the sample and sensing the light scattered from particles of turbidity suspended in the water. When low levels of turbidity are sought to be sensed, the presence of stray light in the instrument becomes more and more critical. The nephelometer, which is designed to reduce the affects of stray light, comprises a transport cell for containing a liquid sample, a light source, and a detector, whereby the cell is characterized in that either the illuminating light beam or the detected light passes through the cell wall face at an acute angle to the normal to the cell wall face.
U.S. Pat. No. 5,589,935 to Biard teaches a turbidity sensor having two light sensitive components, one of which is displaced from a light source, such as a light emitting diode, so that a fluid can pass therebetween, and the other of which is disposed within a common compartment with the light source so that it can measure the intensity of light provided by the light source. A regulator is provided to control the magnitude of current provided to the light source so that its light intensity can be regulated. In this way, the intensity of light emitted by the light source, such as a light emitting diode, can be controlled regardless of the aging of the LED, the variability of LED characteristics and the temperature surrounding the LED. The constant light emission from the light source permits the other light sensitive component to be used as a reliable indication of the turbidity of the solution passing between the light source and the first light sensitive component.
One problem with conventional turbidity sensing systems is the occurrence of scattered signal foldback. Scattered signal foldback occurs when the fluid under evaluation is so turbid that light scattered from the light source cannot make it to the light detector due to further internal scattering, the result of which is a reduction in the effectiveness of the signal.